The present invention relates to an apparatus for the non-destructive, dimensional control of a hollow member. It in particular makes it possible to carry out dimensional controls of machined, hollow members, whose cavity is inaccessible to conventional mechanical means, such as sensors or comparators. It makes it possible to determine the thickness of the wall of said members at any point of their surface.
At present, this type of dimensional control of a machined hollow member is performed by means of ultrasonics. Due to the fact that the transit time of the ultrasonics, having a given velocity, through the wall of a member is a function of the wall thickness, the measurement of this transit time makes it possible to reciprocally deduce therefrom the thickness of the said wall. This ultrasonic control process firstly consists of investigating an average ultrasonic propagation velocity in the wall of the member which it is wished to check, so as to obtain wall thickness values identical to those given by a mechanical means (said measurement taking place on a standard half-member due to the inaccessibility of the cavity to the comparator), then carrying out measurements based on this average velocity. However, the errors regarding the thickness of members eresulting from this method are relatively high, i.e. .+-.0.1 mm for members having a thickness between 5 and 1O mm. This is due to the fact that the production processes of a member lead to structural variations between the individual areas of the member leading to a dispersion of the ultrasonic velocity. The average ultrasonic velocity adopted is consequently not very significant in the areas of the member where there is a high velocity. dispersion.
The ultrasonic control or inspection process which is easy and inexpensive to perform, in most cases makes it possible to determine the thickness of the wall of a machined member, in view of the permitted thickness tolerances, however, it cannot be used for members which are to serve as standards, as a result of the limited accuracy of the measurements.